Known walk-behind working machines include walk-behind lawn mowers. FIG. 23 hereof illustrates, in side elevation, an example of such walk-behind lawn mowers. A walk-behind lawn mower 200 for mowing grass 201 includes an engine 202, a clutch 208 for transmitting a drive power from the engine 202 to rear wheels 209, and a blade 203 to be rotated by the power from the engine 202. The lawn mower 200 also includes a handle 205, a grip 206 attached to the handle 205, and a clutch lever 207 mounted to the handle 205. Front wheels of the lawn mower 200 are designated 210. In mowing the grass 201 using the lawn mower 200, an operator 204 first actuates the engine 202 to thereby rotate the blade 203. The operator 204 then turns the clutch lever 207 forwardly, holding the grip 206 to thereby switch the clutch 208 to an on state in which the drive power of the engine 202 is transmitted via the clutch 208 to the rear wheels 209. This allows the lawn mower 200 to self-propel or travel on flat or sloped ground while mowing the grass 201.
For some applications, the clutch 208 is often a cheap, simply arranged dog clutch which can be rapidly brought into engagement. If the lawn mower 200 employs the dog clutch, the former would suddenly start to move quickly. At this time, the rear wheels 208 can cause its wheelspin. In addition, the front wheels 210 can also be suddenly lifted up together with the blade 203. The wheelspin of the rear wheels 208 would adversely lay or otherwise irregularly cut the grass 201. The dog clutch is not preferred because the lawn mower 200, when starting to move, is difficult to manipulate in such a manner as to satisfactorily mow the grass 201. The operator 204 needs to have high skill in manipulating such a lawn mower.
To solve the above problem, one may propose to provide the lawn mower 200 arranged to travel at a variety of selected velocities. With this arrangement, the lawn mower 200 can begin to move at a low velocity.
More specifically, the thus arranged lawn mower 200 further includes a continuously variable transmission interposed between the engine 202 and the rear wheels 209, and a speed change lever for operating the variable transmission. The operator 204 can change a speed of the lawn mower 200 by handling the speed change lever.
Addition of the variable transmission makes the lawn mower expensive. Moreover, the operator 204 needs to tiresomely handle the speed change lever as well as the clutch lever 207.
Alternatively, the working machine can employ a belt slip clutch including a driving pulley, a driven pulley, and a belt running over the pulleys. Tension in the belt can be varied. This arrangement eliminates the need to provide the costly variable transmission to the lawn mower. This belt slip clutch is designed to be brought to a slipped state in which the belt slips on the pulleys, so that the lawn mower begins to move at a low speed.
The tension in the belt can be varied in correspondence to a force exerted on the clutch lever 207 by the operator 204. The operator 204 can thus bring the clutch into and out of engagement as well as changing the velocity of the lawn mower. When the operator turns forwardly the clutch lever 207, pushing the grip 206, the belt slip clutch is operated such that the lawn mower 200 travels at low or high loads. When “the working machine travels at the low load”, it is meant that the working machine runs on flat ground, for example. When “the lawn mower 200 travels at the high load”, it is meant that the lawn mower 200 runs on sloped ground, for example.
The operator 204 must push the clutch lever 207 with a large force so as to cause the clutch 208 to transmit a maximum power from the engine 202 to the rear wheels 209. It is thus preferred that the lawn mower 200 include a link mechanism etc. for increasing a small force exerted by the operator 204 on the clutch lever 207 so as to achieve the transmission of the maximum power from the engine 202 to the rear wheels 209.
The smaller the force required to achieve the transmission of the maximum power is, however, the smaller a minimum force required to engage the clutch 208 is. It is thus likely that exertion of even a very small force on the lever 207 would bring the clutch into engagement. In other words, the lawn mower 200 would begin to move even when the operator 204 unintentionally lightly touches the clutch lever 207. For this reason, the minimum force is preferably larger in magnitude than a given value.
In order to change the speed of the lawn mower 200, one may further propose to provide the lawn mower 200 with a slip clutch as disclosed, for example, in Japanese Patent Laid-Open Publication No. HEI 3-157520 entitled “SLIP CLUTCH HAVING A SPHERICAL FRICTION SURFACE”.
The disclosed slip clutch includes a slip plate having a concave surface, a friction plate applied to the concave surface of the slip plate, and a pressure plate having a convex surface to be in friction engagement with the friction plate. Where the lawn mower 200 employs the slip clutch, when the slip clutch is engaged, the pressure plate comes into engagement with the friction plate to thereby produce a friction force therebetween, such that a drive power from the engine is transmitted to the rear wheels.
The operator 204 engages or disengages the slip clutch as well as changing the velocity of the lawnmower 200 by pushing the clutch lever 207 with forces of different magnitudes. The operator 204 turns the clutch lever 207 forwardly, pushing the grip 206 to thereby engage the slip clutch, such that the lawn mower 200 travels at the low or high load.
The slip clutch is used for a long time in a slipped state in which the friction plate slips on the pressure plate. The magnitude of the friction force produced between the friction plate and the pressure plate when the clutch began to be used must be maintained for a long period of time.
At a time when the slip clutch begins to be used, the concave surface of the friction plate and the convex surface of the pressure plate provide increased surface roughness. The concave surface and the convex surface therefore have their relatively small areas contacting each other. For the slip clutch which has been used for a long period of time, whereas, the friction plate and the pressure plate are worn. As a result, the worn friction plate and pressure plate undesirably contact each other along the increased surfaces thereof. More specifically, the contact areas of the concave surface of the friction plate and the convex surface of the pressure plate become large. A surface pressure between the friction plate and the pressure plate is smaller when the clutch is used for the long time than when the clutch began to be used even if the magnitude of a force that presses the pressure plate against the friction plate is constant. Consequently, a friction force produced between the pressure plate and the friction plate is small. The slip clutch thus arranged would be less properly operated.
To address this problem, the friction plate and the pressure plate can be produced with increased precision to provide decreased surface roughness, such that the contact area becomes large when the slip clutch begins to be used. However, producing the friction plate and the pressure plate with increased precision requires a high cost.
With the foregoing in view, what has been needed is an inexpressive clutch which can be advantageously used in the slipped state for a long period of time.